The advantage of conveyor belts constituted by spirals is that firstly, they greatly withstand the ingress of dirt since their structure is a smooth, open, monofilament structure and secondly, they withstand flattening, this imparting thereto constant permeability to fluids (in particular air) which would otherwise pass therethrough.
Due to these very advantageous features, such conveyor belts are used in paper-making machines in which, when drying sheets of paper, water vapour is removed which must pass through the conveyor belt.
To properly dry the paper, it is necessary for the permeability to air of the conveyor belt to remain constant.
However, in rapid-operation machines which manufacture ordinary paper, a large boundary layer of air is entrained by the conveyor belts and greatly disturbs the conveying of a sheet from one drying cylinder to another.
To remedy said drawback, it is known to reduce conveyor belt permeability by inserting flat bars inside the spirals. Such a conveyor belt is described e.g. in German Pat. No. 265,673.
The drawback of such a conveyor belt is that after being used for some time, ingress of dirt therein is excessive.
Indeed, impurities are deposited on the bars and in particular along their edges, thereby reducing the permeability of the conveyor belt.